Background EMG Research Articles

Differences in motor inhibition in young and older musicians and non-musicians at rest.

Older adults experience a decline in motor inhibition. These declines have been implicated in instrumental activities of daily living. However, studies have revealed that older musicians have behavioral and neurophysiological enhancements in various motor domains compared to non-musicians. This suggests that music training may delay the decline in motor inhibition with aging. Nevertheless, motor inhibition has not been studied in young or older musicians and non-musicians. Thus, the present study aimed to investigate the neurophysiological differences in motor inhibition in aging musicians and non-musicians. A total of 19 healthy young adult musicians, 16 healthy young non-musicians, 13 healthy older adult musicians, and 16 healthy older adult non-musicians were recruited for the study. Transcranial magnetic stimulation single-pulse (SP) and short interval cortical inhibition (SICI) were performed at rest and then converted into inhibition percentage. We did not observe significant differences between young and older musicians and non-musicians in resting SP MEP. Older adults had lower resting SICI MEP than young adults. Older adults (36%) had a greater percentage of inhibition than young adults (16%). However, when controlling for background EMG activity, musicians had a lower inhibition percentage than non-musicians. The results revealed that, despite the greater use of spinal mechanisms, decreased SICI, and increased inhibition percentage in older adults, motor inhibitory circuitry remains intact and functional in both young and older musicians and non-musicians. Future studies will reveal whether there are differences in motor inhibition during movement in musicians across a person's lifespan.

Dynamic Control of Upper Limb Stretch Reflex in Wrestlers.

ABSTRACTPurposeThe objective of this study was to clarify the characteristics of the upper limb stretch reflex in wrestlers.MethodsTen wrestlers and 11 control subjects participated in the study. The experiment was divided into two sessions. In the extension perturbation session, participants either relaxed or flexed the elbow when they felt a perturbation (abrupt elbow extension induced by a dynamometer). This was done 30 times by each subject for both sessions. In the flexion perturbation session, participants also relaxed or extended the elbow when they felt a perturbation (abrupt elbow flexion). During the tasks, the stretch reflex was monitored by recording the surface EMG activities of the right biceps and triceps brachii. The EMG reflex components were divided into three periods based on the time after the perturbation (M1, 20–50 ms; M2, 50–80 ms; and M3, 80–100 ms). The averaged background EMG activity just before the disturbance was subtracted from the EMG activity in each period. The resultant value was integrated to obtain reflex magnitudes of M1 to M3.ResultsFor the triceps brachii, in the relaxation task, the wrestler group showed a significantly smaller value for M2 than did the control group. In the extension task, the wrestler group showed a significantly larger value for M3 than did the control group. There was no difference in M1 between the two groups. For the biceps brachii, there was no significant difference between any reflex components.ConclusionsOur results suggest that high-level wrestlers have specific characteristics of the long-latency stretch reflex in the triceps brachii that are modulated in a situation-specific manner.

Modulation of cutaneous reflexes during sidestepping in adult humans.

A common neural control mechanism coordinates various types of rhythmic locomotion performed in the sagittal plane, but it is unclear whether frontal plane movements show similar neural patterning in adult humans. The purpose of this study was to compare cutaneous reflex modulation patterns evoked during sagittal and frontal plane rhythmic movements. Eight healthy, neurologically intact adults (three males, five females) walked and sidestepped on a treadmill at approximately 1Hz. The sural nerve of the dominant (and lead) limb was stimulated randomly every 3-7 steps at eight phases of each gait cycle. Ipsilateral electromyographic recordings from four lower leg muscles and kinematic data from the ankle were collected continuously throughout both tasks. Data from unstimulated gait cycles were used as control trials to calculate middle-latency reflex responses (80-120ms) and kinematic changes (140-220ms) following electrical stimulation. Results show that the cutaneous reflex modulation patterns were similar across both tasks despite significant differences in background EMG activity. However, increased reflex amplitudes were observed during the late swing and early stance phases of sidestepping, which directly altered ankle kinematics. These results suggest that the neural control mechanisms responsible for coordinating sagittal locomotion are flexibly modified to coordinate frontal plane activities even with very different foot landing mechanics.

Modulation of soleus muscle H-reflexes and ankle muscle co-contraction with surface compliance during unipedal balancing in young and older adults

This study aimed to assess modulation of lower leg muscle reflex excitability and co-contraction during unipedal balancing on compliant surfaces in young and older adults. Twenty healthy adults (ten aged 18–30 years and ten aged 65–80 years) were recruited. Soleus muscle H-reflexes were elicited by electrical stimulation of the tibial nerve, while participants stood unipedally on a robot-controlled balance platform, simulating different levels of surface compliance. In addition, electromyographic data (EMG) of soleus (SOL), tibialis anterior (TA), and peroneus longus (PL) and full-body 3D kinematic data were collected. The mean absolute center of mass velocity was determined as a measure of balance performance. Soleus H-reflex data were analyzed in terms of the amplitude related to the M wave and the background EMG activity 100 ms prior to the stimulation. The relative duration of co-contraction was calculated for soleus and tibialis anterior, as well as for peroneus longus and tibialis anterior. Center of mass velocity was significantly higher in older adults compared to young adults (p<0.001) and increased with increasing surface compliance in both groups (p<0.001). The soleus H-reflex gain decreased with surface compliance in young adults (p= 0.003), while co-contraction increased {(p}_{mathrm{S}mathrm{O}mathrm{L},mathrm{T}mathrm{A}}=0.003 mathrm{a}mathrm{n}mathrm{d} {p}_{mathrm{P}mathrm{L},mathrm{T}mathrm{A}}<0.001). Older adults did not show such modulations, but showed overall lower H-reflex gains (p<0.001) and higher co-contraction than young adults {(p}_{mathrm{S}mathrm{O}mathrm{L},mathrm{T}mathrm{A}}<0.001 mathrm{a}mathrm{n}mathrm{d} {p}_{mathrm{P}mathrm{L},mathrm{T}mathrm{A}}=0.002). These results suggest an overall shift in balance control from the spinal level to supraspinal levels in older adults, which also occurred in young adults when balancing at more compliant surfaces.

Neural mechanism of selective finger movement independent of synergistic movement.

Muscle synergy is important for simplifying functional movement, which constitutes spatiotemporal patterns of activity across muscles. To execute selective finger movements that are independent of synergistic movement patterns, we hypothesized that inhibitory neural activity is necessary to suppress enslaved finger movement caused by synergist muscles. To test this hypothesis, we focused on a pair of synergist muscles used in the hand opening movement, namely the index finger abductor and little finger abductor (abductor digiti minimi; ADM), and examined whether inhibitory neural activity in ADM occurs during selective index finger abduction/adduction movements and/or its imagery using transcranial magnetic stimulation and F-wave analysis. During the index finger adduction movement, background EMG activity, F-wave persistence, and motor evoked potential (MEP) amplitude in ADM were elevated. However, during the index finger abduction movement, ADM MEP amplitude remained unchanged despite increased background EMG activity and F-wave persistence. These results suggest that increased spinal excitability in ADM is counterbalanced by cortical-mediated inhibition only during selective index finger abduction movement. This assumption was further supported by the results of motor imagery experiments. Although F-wave persistence in ADM increased only during motor imagery of index finger abduction, ADM MEP amplitude during motor imagery of index finger abduction was significantly lower than that during adduction. Overall, our findings indicate that cortical-mediated inhibition contributes to the execution of selective finger movements that are independent of synergistic hand movement patterns.

Transspinal stimulation decreases corticospinal excitability and alters the function of spinal locomotor networks.

Locomotion requires the continuous integration of descending motor commands and sensory inputs from the legs by spinal central pattern generator circuits. Modulation of spinal neural circuits by transspinal stimulation is well documented, but how transspinal stimulation affects corticospinal excitability during walking in humans remains elusive. We measured the motor evoked potentials (MEPs) at multiple phases of the step cycle conditioned with transspinal stimulation delivered at sub- and suprathreshold intensities of the spinally mediated transspinal evoked potential (TEP). Transspinal stimulation was delivered before or after transcranial magnetic stimulation during which summation between MEP and TEP responses in the surface EMG was absent or present. Relationships between MEP amplitude and background EMG activity, silent period duration, and phase-dependent EMG amplitude modulation during and after stimulation were also determined. Ankle flexor and extensor MEPs were depressed by suprathreshold transspinal stimulation when descending volleys were timed to interact with transspinal stimulation-induced motoneuron depolarization at the spinal cord. MEP depression coincided with decreased MEP gain, unaltered MEP threshold, and unaltered silent period duration. Locomotor EMG activity of bilateral knee and ankle muscles was significantly depressed during the step at which transspinal stimulation was delivered but fully recovered at the subsequent step. The results support a model in which MEP depression by transspinal stimulation occurs via subcortical or spinal mechanisms. Transspinal stimulation disrupts the locomotor output of flexor and extensor motoneurons initially, but the intact nervous system has the ability to rapidly overcome this pronounced locomotor adaptation. In conclusion, transspinal stimulation directly affects spinal locomotor centers in healthy humans.NEW & NOTEWORTHY Lumbar transspinal stimulation decreases ankle flexor and extensor motor evoked potentials (MEPs) during walking. The MEP depression coincides with decreased MEP gain, unaltered MEP threshold changes, and unaltered silent period duration. These findings indicate that MEP depression is subcortical or spinal in origin. Healthy subjects could rapidly overcome the pronounced depression of muscle activity during the step at which transspinal stimulation was delivered. Thus, transspinal stimulation directly affects the function of spinal locomotor networks in healthy humans.

Absence of joint pain identifies high levels of sleep masticatory muscle activity in myofascial temporomandibular disorder.

Although the development of reliable diagnostic criteria for temporomandibular disorders (TMDs) has operationalised identification of a subgroup with myofascial pain (mTMD), causal mechanisms remain elusive. This study examines masticatory muscle activity (MMA) in more homogenous research subgroups of mTMD. Data from an existing case-control study of women were used to subcategorise mTMD cases based on joint pain with palpation to isolate muscle-only pain (M-pain) vs muscle and joint pain (MJ-pain). Differences in laboratory indicators of MMA, specifically research diagnostic criteria for sleep bruxism (SB) and high background EMG activity, and other clinical and sociodemographic indicators were examined between groups. Compared to controls, the MJ-pain subgroup did not show elevated background EMG or sleep bruxism. In contrast, the M-pain subgroup showed significantly higher background EMG and a trend towards elevated prevalence of sleep bruxism. These results may explain why it has been difficult for studies of SB in mixed TMD and even mTMD samples to find a consistent positive association, since a positive association may be limited to mTMD without joint pain. The subcategorising of mTMD based on joint pain with palpation (ie M-pain, MJ-pain) appears to reveal subgroups with relatively high and low sleep masticatory muscle-specific activity. Findings need replication in a larger study with updated mTMD diagnostic criteria, but may prove useful for understanding mechanism of pain maintenance in mTMD with and without joint pain.

P32-S Application of decomposition based quantitative EMG (DQEMG) to focal neuropathies

Background EMG signals can be decomposed into their constituent motor unit potential trains and further analyzed using specific signal processing algorithms which provide novel and useful information regarding the temporal and electrophysiological behaviour and morphological attributes of motor units. In low to moderate stages of focal neuropathies (when only focal demyelination or low axonal sensory affection is registered) traditional needle EMG evaluation is usually normal. However, DQEMG provides evidence of motor unit instability in a significant portion of otherwise normal muscles. Material and methods 20 case and 9 control subjects were studied with median and ulnar ENG, quantitative EMG and DQEMG (abductor pollicis brevis, first dorsal interosseus or abductor digiti minimi). Patients with symptoms suggesting focal median or ulnar neuropathies were selected as cases, except for those showing low-median compound muscle action potential amplitudes or needle EMG abnormalities. Control subjects were included when no ENG nor needle EMG abnormalities findings were encountered. Results 75% of cases with any focal demyelinating finding showed instability features in DQEMG, which were also found in 43% of the normal ENG and control groups. Applying more restrictive DQEMG criteria, instability was found in 63% of cases with focal ENG alterations and 43% with normal ENG, while the control group showed no abnormalities. Conclusions Decomposition-based quantitative EMG is a novel and more powerful tool for detecting abnormalities in motor unit potentials, compared with the classical quantitative or semi-quantitative needle EMG analysis.

Modulation of tendon tap reflex activation of soleus motor neurons with reduced stability tandem stance

Reduced stability while standing typically decreases the soleus muscle Hoffmann (H-) reflex amplitude, purportedly to prevent the Ia afferent signal from excessively activating spinal motor neurons during the unstable stance. H-reflex measures, however, by excluding the spindle do not reflect the actual effect of the Ia pathway (i.e. the combined effects of spindle sensitivity and Ia presynaptic inhibition) on motor neuron activation, as tendon tap reflex measures can. But the effect of stance stability on soleus muscle tendon tap reflex amplitude is largely unknown. This study examined 30 young adults (mean(s), 21(2) years) as they stood in a wide stable stance position and an unstable tandem stance with a reduced base of support. Standing body sway, the amplitude of the soleus muscle tendon tap reflex, background EMG and tap force were measured in both stances. A repeated measured design t-test was calculated for each variable. Most subjects (69%) decreased tendon tap reflex amplitude when in the tandem stance position (mean decrease 11.6%), compared to the wide stance (wide stance 0.248(0.124) mV, tandem stance 0.219(0.119) mV, p < 0.05, Cohen’s d = 0.24 small) with no significant differences in background soleus and tibialis anterior EMG, and tap force across the stances. There was no relationship between the modulation of the tendon tap reflex amplitude across the stances and standing body sway in the tandem stance. Results support the idea that for most subjects examined, during a less stable stance the Ia excitation of motor neurons is decreased, likely by presynaptic inhibition, thereby avoiding potential instability in the reflex loop or saturating the reflex pathway and possibly interfering with descending control of the involved spinal motor neurons.

Corticospinal excitability during shortening and lengthening actions with incremental torque output.

What is the central question of this study? The relationship between motor unit recruitment and firing rate has been related to the size of the corticospinal output with variations in the nervous system gain during isometric contractions. However, corticospinal behaviour with incremental torque output might differ during anisometric contractions owing to differences in neural control of anisometric contraction types. What is the main finding and its importance? Corticospinal excitability during lengthening contractions was smaller compared with shortening but increased with incremental torque output in a similar manner between contraction types. The relationship between motor unit recruitment and firing rates is probably the main determinant of the size of an evoked response with variations in system gain. The modulation of motor evoked potentials (MEPs), an index of corticospinal excitability, has been shown to increase during isometric contractions with incremental torque output in accordance with the contribution between motor unit recruitment and firing rate of the muscle to increases in required torque output. However, the motor unit strategy of the muscle might not be the only factor influencing this behaviour, because differences in pre- and postsynaptic control have been reported between lengthening and shortening or isometric contractions. In 30 healthy adults, MEPs were elicited in tibialis anterior during shortening and lengthening contractions at 15, 25, 50 and 80% of contraction-type-specific maximal voluntary contraction torque. Background EMG activity increased progressively with greater torque output (P<0.001) but was similar between contraction types (P=0.162). When normalized to the maximal muscle response, MEPs were greater during shortening compared with lengthening contractions (P=0.004) and increased stepwise with increased contraction intensities (P=0.001). These data show an increase in corticospinal excitability with torque output from lower to higher contraction intensities, suggesting a greater contribution of motor unit recruitment to increased nervous system gain in the tibialis anterior. Despite differences in corticospinal control of shortening and lengthening contractions, the data suggest that the corticospinal responses to increases in torque output are not dependent on contraction type, because corticospinal excitability increased to a similar extent during shortening and lengthening actions. Thus, it is likely that the relationship between motor unit recruitment and firing rate of the muscle is the main determinant of corticospinal output with variations in nervous system gain.

Age-related reversal of spinal excitability during anticipatory postural control

IntroductionAn internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs). In healthy younger adults, a measure of spinal excitability in the form of the Hoffmann (H) reflex becomes depressed during APAs but how aging affects the reflex control of APAs is unknown.MethodsWe compared H reflex excitability profiles in the right soleus muscle, indirectly indicating APA, between younger (n = 11, age 19–24 years), middle-aged (n = 10, age 37–56 years), and older healthy adults (n = 11, age 63–78 years). Subjects rapidly raised the right-dominant arm in response to an auditory cue. The H reflex was evoked 120 ms, 100 ms, 80 ms, 60 ms, 40 ms, 20 ms, and 0 ms before as well as 20 ms after the onset of the right anterior deltoid muscle activation. For data processing, each trial was controlled for the corresponding background EMG activity before normalizing the standing data to the data in sitting in the 8 time bins.ResultsAll subjects showed a silent period in the soleus background electromyographic activity, suggesting the presence of APA. We found that the stereotypical H reflex depression associated with APAs in younger adults was reduced in middle-aged adults and reversed to facilitation in older adults. The depression occurred in 10 out of 11 younger adults, whereas all 11 older adults exhibited facilitation.ConclusionBecause APAs are organized at the supraspinal level, we speculate a supraspinal origin of the age-related reflex facilitation during APAs.

Operant Up-Conditioning of the Tibialis Anterior Motor-Evoked Potential in Multiple Sclerosis: Feasibility Case Studies.

Damage to the corticospinal pathway often results in weak dorsiflexion of the ankle, thereby limiting the mobility of people with multiple sclerosis (MS). Thus, strengthening corticospinal connectivity may improve locomotion. Here, we investigated the feasibility of tibialis anterior (TA) motor-evoked potential (MEP) operant conditioning and whether it can enhance corticospinal excitability and alleviate locomotor problems in people with chronic stable MS. The protocol consisted of 6 baseline and 24 up-conditioning sessions over 10 weeks. In all sessions, TA MEPs were elicited at 10% above active threshold while the sitting subject provided 30–35% maximum voluntary contraction (MVC) level of TA background EMG. During baseline sessions, MEPs were simply measured. During conditioning trials of the conditioning sessions, the subject was encouraged to increase MEP and was given immediate feedback indicating whether MEP size was above a criterion. In 3/4 subjects, TA MEP increased 32–75%, MVC increased 28–52%, locomotor EMG modulation improved in multiple leg muscles, and foot drop became less severe. In one of them, MEP and MVC increases were maintained throughout 3 years of extensive follow-up sessions. These initial results support a therapeutic possibility of MEP operant conditioning for improving locomotion in people with MS or other CNS disorders, such as spinal cord injury and stroke.

Effects of posture and coactivation on corticomotor excitability of ankle muscles.

The use of transcranial magnetic stimulation (TMS) to evaluate corticomotor excitability of lower limb (LL) muscles can provide insights about neuroplasticity mechanisms underlying LL rehabilitation. However, to date, a majority of TMS studies have focused on upper limb muscles. Posture-related activation is an important under-investigated factor influencing corticomotor excitability of LL muscles. The purpose of this study was to evaluate effects of posture and background activation on corticomotor excitability of ankle muscles. Fourteen young neurologically-unimpaired participants (26.1±4.1 years) completed the study. TMS-evoked motor evoked potentials (MEPs) were recorded from the tibialis anterior (TA) and soleus during 4 conditions - standing, standing coactivation, sitting, and sitting coactivation. TA and soleus MEP amplitudes were compared during: (1) standing versus sitting;(2) standing coactivation (standing while activating both TA and soleus) versus sitting coactivation; and (3) standing coactivation versus standing. For each comparison, background EMG for TA and soleus were matched. Trial-to-trial coefficient of variation of MEP amplitude and coil-positioning errors were additional dependent variables. No differences were observed in TA or soleus MEP amplitudes during standing versus sitting. Compared to sitting coactivation, larger MEPs were observed during standing coactivation for soleus but not TA. Compared to standing, the standing coactivation task demonstrated larger MEPs and reduced trial-to-trial MEP variability. Our findings suggest that incorporation of measurements in standing in future TMS studies may provide novel insights into neural circuits controlling LL muscles. Standing and standing coactivation tasks may be beneficial for obtaining functionally-relevant neuroplasticity assessments of LL musculature.

Cervical Vestibular Evoked Myogenic Potentials (cVEMPs) Evoked by Air-Conducted Stimuli in Patients with Functional Neck Dissection

ObjectiveCervical vestibular-evoked myogenic potentials (cVEMPs) are short-latency vestibulocollic reflexes. The damage on any point of the vestibulocollic reflex pathway could affect the cVEMPs. Whether neck dissection has an effect on the sacculocollic pathway, and consequently on cVEMPs, remains unexplored. The aim of this study was to evaluate the cVEMP findings in patients with functional neck dissection without vestibular symptoms. Patients and methodsThis cross-sectional study design, 20 patients who had undergone unilateral neck dissection with sparing of the accessory nerve, SCM and internal jugular vein, were included. The response rates (%), cVEMPs parameters such as the prestimulus background EMG [Root mean square (RMS)] activity (μV), P13 and N23 peak latencies (ms), interpeak (N23–P13) interval (ms), scale and non-scale interpeak (N23–P13) amplitudes (μV) were compared between the groups. Amplitude asymmetry ratio (AAR) was calculated. ResultsTwenty patients (14 males and 6 females), age was between 38 and 79 years were included in the study. All of the patients had clear cVEMPs on the NOS, whereas 18/20 (90%) patients had on the NDS. P13 and N23 peak latency of the NDS were found to be significantly longer than the NOS (P = 0.01). There was no significant difference in N23-P13 interpeak interval between two sides (P > 0.05). There was a negative correlation between P13 peak latency and post-operative time (P = 0.042; R = ‑0.484). Scale and non-scale N23–P13 interpeak amplitudes of the NDS were found to be significantly lower than the NOS (P = 0.03). Mean AAR was found as 0.28 ± 0.16 (0.08–0.76). Seven patients (35%) had abnormal amplitude asymmetry. RMS values, were statistically and significantly lower in NDS compared to NOS (P = 0.01). However, no correlation was observed between the RMS values and peak latency and peak amplitude values (P > 0.05). ConclusionscVEMP testing is an easy-to-apply, non-invasive, painless, and recordable test that can be used for evaluations of SAN and SCM function for patients undergoing neck dissection. After neck dissection, VEMP abnormalities can be detected. However, further studies are needed to indicate whether these abnormalities originate within the vestibular system and are due to pathologies originating from the SANs and SCMs. In addition, preoperative and postoperative studies are needed to better guide the clinical application of cVEMP testing.

Threshold position control of anticipation in humans: a possible role of corticospinal influences.

Sudden unloading of preloaded wrist muscles elicits motion to a new wrist position. Such motion is prevented if subjects unload muscles using the contralateral arm (self-unloading). Corticospinal influences originated from the primary motor cortex maintain tonic influences on motoneurons of wrist muscles before sudden unloading but modify these influences prior to the onset and until the end of self-unloading. Results are interpreted based on the previous finding that intentional actions are caused by central, particularly corticospinal, shifts in the spatial thresholds at which wrist motoneurons are activated, thus predetermining the attractor point at which the neuromuscular periphery achieves mechanical balance with environment forces. By maintaining or shifting the thresholds, descending systems let body segments go to the equilibrium position in the respective unloading tasks without the pre-programming of kinematics or muscle activation patterns. The study advances the understanding of how motor actions in general, and anticipation in particular, are controlled. The role of corticospinal (CS) pathways in anticipatory motor actions was evaluated using transcranial magnetic stimulation (TMS) of the primary motor cortex projecting to motoneurons (MNs) of wrist muscles. Preloaded wrist flexors were suddenly unloaded by the experimenter or by the subject using the other hand (self-unloading). After sudden unloading, the wrist joint involuntarily flexed to a new position. In contrast, during self-unloading the wrist remained almost motionless, implying that an anticipatory postural adjustment occurred. In the self-unloading task, anticipation was manifested by a decrease in descending facilitation of pre-activated flexor MNs starting ∼72ms before changes in the background EMG activity. Descending facilitation of extensor MNs began to increase ∼61ms later. Conversely, these influences remained unchanged before sudden unloading, implying the absence of anticipation. We also tested TMS responses during EMG silent periods produced by brief muscle shortening, transiently resulting in similar EMG levels before the onset and after the end of self-unloading. We found reduced descending facilitation of flexor MNs after self-unloading. To explain why the wrist excursion was minimized in self-unloading due to these changes in descending influences, we relied on previous demonstrations that descending systems pre-set the threshold positions of body segments at which muscles begin to be activated, thus predetermining the equilibrium point to which the system is attracted. Based on this notion, a more consistent explanation of the kinematic, EMG and descending patterns in the two types of unloading is proposed compared to the alternative notion of direct pre-programming of kinematic and/or EMG patterns.

Vibro-tactile and auditory balance biofeedback changes muscle activity patterns: Possible implications for vestibularimplants.

The two different types of balance prostheses being developed, implants and vibro-tactile/auditory feedback prostheses, rely on different measures to prove efficacy (those based on vestibular ocular reflexes versus balance control, respectively). Here we provide evidence that examining muscle activity might provide a useful alternative for both. The muscle activity of 6 bilateral vestibular loss (BVL) and 7 age-matched healthy controls (HC) was examined while standing eyes closed on a foam support surface. Pelvis and upper trunk angular movements were recorded in the roll and pitch planes. Surface EMG was recorded from the lower leg, trunk and upper arm muscles. BVL subjects were first assessed without feedback of pelvis sway, then received training with combined vibro-tactile and auditory feedback, before being re-assessed with feedback. Feedback reduced the amplitudes of pelvis and shoulder sway to values of HC without feedback. Both the level of background EMG activity and the EMG area amplitudes changed when feedback was provided in a manner consistent with the reduced amplitude modulation of muscle synergies of HC. The results of this study indicate that changed muscle synergy amplitudes underlie improvements in sway achieved by BVL subjects. The concept of this investigation may provide a means to prove efficacy for different types of balance prostheses, including implants.

Enhanced Corticospinal Excitability and Volitional Drive in Response to Shortening and Lengthening Strength Training and Changes Following Detraining.

There is a limited understanding of the neurological adaptations responsible for changes in strength following shortening and lengthening resistance training and subsequent detraining. The aim of the study was to investigate differences in corticospinal and spinal responses to resistance training of the tibialis anterior muscle between shortening or lengthening muscle contractions for 4 weeks and after 2 weeks of detraining. Thirty-one untrained individuals were assigned to either shortening or lengthening isokinetic resistance training (4 weeks, 3 days/weeks) or a non-training control group. Transcranial magnetic stimulation and peripheral nerve stimulation (PNS) were used to assess corticospinal and spinal changes, respectively, at pre-, mid-, post-resistance training and post detraining. Greater increases changes (P < 0.01) in MVC were found from the respective muscle contraction training. Motor evoked potentials (expressed relative to background EMG) significantly increased in lengthening resistance training group under contraction intensities ranging from 25 to 80% of the shortening and lengthening contraction intensity (P < 0.01). In the shortening resistance training group increases were only seen at 50 and 80% of both contraction type. Volitional drive (V-wave) showed a greater increase following lengthening resistance training (57%) during maximal lengthening contractions compared to maximal shortening contractions following shortening resistance training (23%; P < 0.001). During the detraining period MVC and V-wave did not change (P > 0.05), although MEP amplitude decreased during the detraining period (P < 0.01). No changes in H-reflex were found pre to post resistance training or post detraining. Modulation in V-wave appeared to be contraction specific, whereby greatest increases occurred following lengthening resistance training. Strength and volitional drive is maintained following 2 weeks detraining, however corticospinal excitability appears to decrease when the training stimulus is withdrawn.

The role of the supporting lower limb function in the premotor changes in the H-reflex of m. soleus

In studies on healthy humans we examined changes in the H reflex amplitude and the intensity of background EMG of the soleus muscle within the premotor period of voluntary extension of the contralateral ankle joint. The tested persons were in the lying prone position with relaxed legs, or in the standing position with support on both feet or only on one foot with additional hand support. The soleus H reflex was induced using the standard technique by transcutaneous stimulation (1 ms long current pulses) of the tibial nerve afferents in the region of the popliteal dimple. Using superficial electrodes, we recorded the integral EMG reflex discharge from the m. soleus, amplified, and visualized it using a two-channel digital oscillograph, Handiscope HS3 (TiePieEngeneering, Netherlands). In other tests, we also recorded tonic background EMG activity from the soleus muscle. After fullwave rectification and low-frequency filtration we estimated the area under the integral curve enveloping oscillations of the above mentioned EMG. In the lying position the extension of contralateral ankle joint evoked anticipatory facilitation of the testing soleus H-reflex. It manifested in 90–60 ms and reached the maximum 30 ms before the conditioning movement. Considerable changes of the soleus background EMG-activity did not occur during the premotor period. Consequently, it is supposed that the mechanism leading to the H-reflex facilitation is the decrease of presynaptic inhibition at 1a. In the standing position with support on both feet contralateral ankle joint extension caused anticipatory inhibition of the soleus H-reflex and simultaneous suppression of background EMG 90–60 ms before the conditioning movement. These results indicate the anticipatory changes of muscle tone and suggest participation of postsynaptic mechanism in the inhibition of the testing H-reflex. When the moving contralateral extremity had no support, the contralateral ancle joint extension did not cause anticipatory inhibition but facilitation of the soleus H-reflex. Its time course was similar to that observed in the lying position. The considerable changes of the soleus background EMG-activity did not arise. The results of the experiments indicate that the nature of premotor changes in the spinal neuron circuits, associated with the lower limb muscles, depends not only on the type of upcoming voluntary movement. Being caused by the contralateral limb movements, they depend greatly on the fulfillment or nonfulfillment of the support function by the lower limbs.

ID 323 – Shortening of the cutaneous silent period duration in tibialis anterior after fatiguing eccentric exercise

Objective Muscle afferent feedback (presumably mainly of spindle origin) contributes to the maintenance of alpha motoneuron firing during sustained muscle contraction. We aimed at investigating how a unilateral fatiguing eccentric exercise involving the tibialis anterior (TA) affects inhibitory control of muscle activity. Methods In 8 healthy subjects, we assessed the TA silent period (SP) to cortical magnetic stimuli (round coil) and to cutaneous nerve electrical stimulation (superficial peroneal nerve at the ankle) before and after 5 min of eccentric exercise. Results At baseline, SP duration was variable among subjects but cortical SP was always longer than cutaneous SP (111.6 ± 13.8 ms vs. 37.3 ± 10.2 ms). After exercise, there was a slight increase in background EMG activity. The cutaneous SP shortened significantly (24.3 ± 7.3 ms) at the expenses of onset latency delay (98.7 ± 14.0 ms in baseline to 112.2 ± 11.9 ms after exercise). There were no significant changes in cortical SP after exercise (106.2 ± 18.1 ms). Conclusions Eccentric exercise leads to a delay in onset latency of the cutaneous SP with no changes in cortical SP. An effect of fatigue on small nerve fibers, which are known to contribute to the onset of cutaneous SP, may partly explain our results.

The relationship between dynamic balancing ability and posture-related modulation of the soleus H-reflex

Soleus H-reflex reveals down modulation with increased postural difficulty. Role of this posture-related reflex modulation is thought to shift movement control toward higher motor centers in order to facilitate more precise postural control. Present study hypothesized that the ability to modulate H-reflex is related to one’s ability to dynamically balance while in an unstable posture. This study examined the relationship between dynamic balancing ability and soleus H-reflex posture-related modulation. Thirty healthy adults participated. The soleus maximal H-reflex (Hmax), motor response (Mmax), and background EMG activity (bEMG) were obtained during three postural conditions: prone, open-legged standing, and closed-legged standing. Hmax/Mmax ratios were normalized via the corresponding bEMG in order to remove the effects of background muscle activity from the obtained H-reflex. Reflex modulation was calculated as the ratio of the normalized Hmax/Mmax ratios in one postural condition to another posture in a more difficult condition. Dynamic balancing ability was assessed by testing stability while standing on a wobble board. A significant negative correlation was observed between balancing scores and reflex modulation from open-legged standing to closed-legged standing. This suggests that the ability to modulate monosynaptic stretch reflex excitability in response to a changing posture is a significant factor for dynamic balancing.